1. Field of the Invention
This invention relates to generating laser pulses, and more particularly to generating output laser pulses having any wavelength in a nominal range of two to six microns.
2. Discussion of Prior Art
Considerable effort has been invested over the past twenty years in generating continuously tunable infrared (IR) laser emission using optical parametric oscillators (OPO's). In the beginning it was thought that OPO's would quickly evolve to provide wavelength coverage throughout the mid IR. However, many years later problems remain and, in particular, generation of powerful laser pulses in the important 3 micron to 6 micron band has seen only very limited success.
An OPO is pumped with a fixed laser wavelength, typically 532 or 1060 nm, and uses a variable frequency shift to obtain tunable laser emission at a longer wavelength. For example, there are teachings of tuning ranges of 1.29 to 1.44 microns, of tuning ranges of 1.83 to 1.91 microns, and such ranges of 3.5 & 4 to 5 microns.
Considering pump lasers, OPO's are pumped with a fixed wavelength pump laser and thus can use the very well-developed, high gain Nd:YAG medium or possibly a Tm:YAG or Ho:YAG laser. However, OPO's provide a tunable wavelength shift in a nonlinear shifting crystal. Major problems are encountered with nonlinear crystals for OPO's, for example, absorption, phase matching, and optical damage. In Applicants' experience, due to these problems there is no nonlinear crystal which can efficiently convert 1.06 micron pulses to the 3.4 micron to 6 micron band. For example, a shift from 1.064 microns to 3.2 microns and a rapid fall-off of transmission above 3.1 microns have been reported, In Applicants' experience, to reach the 3.4 to 6 micron band one must simultaneously use two sequential OPO's. Alternatively, one must start with a 2 micron Tm:YAG laser. However, currently available Tm:YAG lasers generate much lower average powers than the power available from Nd:YAG lasers. Finally, considering conversion efficiency, commercial OPO systems generate about 2 mJ pulses at 3 microns and less powerful pulses at longer wavelengths. Beyond 4 microns the available OPO systems produce much less than 1 mJ per pulse.
In Applicants' experience, prior laser systems have limited ability to generate output laser pulses that are rapidly tunable to any wavelength in the range of nominally 2 to 6 microns. Problems with OPO's include crystal damage at high powers, thermal lensing in the crystals, inherently large and complex optical chains, and the lack of suitable high-efficiency crystals for converting common lasers to the 3 micron region and longer wavelengths. Frequency doubling carbon dioxide and carbon monoxide laser energy to the mid-infrared limits the wavelengths available to those which are shiftable from known strong carbon dioxide or carbon monoxide lines. Thus infinite laser tunability is not possible by frequency doubling gas phase lasers. Lead salt lasers operate directly in the 2-5 micron band but have very low power (a few milliwatts) and require cryogenic cooling to operate.